PTL 1 discloses that a transparent piezoelectric film speaker is curved and attached to the screen of a mobile phone so that the sounds are output from a wide area and the hearing property from the speaker is improved. In a specifically disclosed method which is suggested, a surface of one of rectangular PVDF (polyvinyl vinylidene fluoride) films in the thickness direction and a surface of the other PVDF film are laminated in such a manner that the surfaces have opposite expansion and contraction behaviors when an electric charge is applied (so-called bimorph structure), and the two short sides are fixed. Sounds are produced by the vibrations generated by curving the films.
PTL 2 proposes a piezoelectric speaker in which an active electrode area is provided on a polymer piezoelectric sheet in the direction along the main surface and the active electrode area is sectioned. By applying electric charges to the active electrode area in such a manner that the electric field vectors generated in neighboring active electrode sections in the thickness direction of the piezoelectric sheet are opposite to each other, even when the four sides of a square piezoelectric sheet are fixed, the piezoelectric sheet warps and sounds can be output. In addition, an L-polylactic acid, which is a chiral polymer, is proposed as the polymer constituting the piezoelectric sheet.
Moreover, in PTL 3 and PTL 4, the present inventors have suggested that the displacement force can be increased by laminating layers made of a poly-L-lactic acid and a poly-D-lactic acid.
Here, in order to bond a piezoelectric element to a diaphragm and produce a sound, there are two systems: a system in which the expansion and contraction displacement of the piezoelectric element warps the diaphragm and the curving vibration produces a sound, and a system in which in-plane expansion and contraction of the piezoelectric element produces an in-plane vibration in the adhered diaphragm and the resonance produces a sound. Because a piezoelectric element made of a polymer has a lower piezoelectric modulus and a weaker force than a piezoelectric ceramic such as PZT, such a piezoelectric element is not suitable for resonating a hard diaphragm, and the system using the curving vibrations of the piezoelectric element has been used as in PTLs 1 and 2.